Genes and Infidelity
New research is also suggesting that genes play a role in the complex story of infidelity. In a study of 7,400 Finnish twins published last year in Evolution and Human Behavior, researchers found that about 10% of men and 6% of women had two or more sexual partners in the last year. They also found a significant association with a gene for vasopressin and the likelihood of infidelity – but they only found this association in women. This hardly makes for a compelling case that infidelity is genetically determined, but it does fit in with a body of animal evidence that supports a relationship between our genes and our ability to be monogamous.
Some of this work comes from studies of two kinds of vole: the montane vole, which is sexually promiscuous, and the prairie vole, which is monogamous. Each of these species has vasopressin receptors that are located in different regions of the brain, and injecting the hormone vasopressin into the little vole brain causes two distinct kinds of behavioral response. When the monogamous prairie vole brain is injected with vasopressin it triggers pair bonding. Blocking of the receptors has an opposite effect – prairie voles still want vole sex, but are no longer monogamous.
Now here’s the crucial anatomical piece: The vasopressin receptors in the monogamous prairie voles lies near the brain’s reward center, so when vasopressin is released, it activates neurons in that reward center. However, the vasopressin receptors in the promiscuous montane voles is found in the amygdla, which is thought to process anxiety and fear. So releases of vasopressin in the promiscuous vole does not stimulate the reward center, and does not lead to pair bonding.
Another team working with twins was not been able to find an association between human infidelity and the vasopressin receptor gene implicated in the sexual behavior of other mammals. However, even that team concluded that "...infidelity and number of sexual partners are both under moderate genetic influence (41% and 38% heritable, respectively) and the genetic correlation between these two traits is strong (47%)." We are certainly very far from identifying a gene for infidelity, but there is evidence that genetics and neuroendocrine release plays some role in the expression of this behavior.
Data From a Website You Should Not Visit
There is new data to support the assertion that married men are more likely to cheat than are married women, and that data came from the Ashley Madison breach. (Now for those of you who did not read about this from the sewers of the Internet, here’s what you need to know. Ashley Madison is a website that offers to pair up married people – men and women – who want to cheat on their spouse and have an affair. In August of 2015 it was revealed that the website had been hacked and some of the data of the subscribers to the service was leaked. OK, that’s the background. No read on to the more important part of the story.)
In a report published in Gizmodo, it was revealed that out of a database of 37 million Ashley Madison users, only about 5.5 million were marked as female. While acknowledging that some of these users are not real, the raw numbers show that for every married woman looking to have an affair, there were more than five married men. The report goes on to note that
...out of 5.5 million female accounts, roughly zero percent had ever shown any kind of activity at all, after the day they were created...The men’s accounts tell a story of lively engagement with the site, with over 20 million men hopefully looking at their inboxes, and over 10 million of them initiating chats. The women’s accounts show so little activity that they might as well not be there….we’re left with data that suggests Ashley Madison is a site where tens of millions of men write mail, chat, and spend money for women who aren’t there.
Jewish Adultery In the Middle Ages
In his essay on rabbinic attitudes towards nonobservance, Ephriam Kanarfogel pointed out that "[s]exual promiscuity and even adultery were never absent from any region on the medieval Jewish world." These adulterous relationships were "widespread", but, continues Kanerfogel, "...the presence of even more objectionable possibilities (i.e., relations with married Jewish women) also had to be considered…" As evidence of just how widespread was the practice of married Jewish men having affairs, Kanarfogel cites R. Moses of Coucy of Spain who preached "at length" in 1236 about the sins of sexual relations outside of marriage. The issue of sexual promiscuity was so widespread that in Toledo a herem (communal ban) was issued against it in 1281. Remarkably, "many who had vowed to honor the ban could not retain themselves and either openly flouted the ban or attempted to circumvent it." It was the widespread promiscuity of married Jewish men that, according to Kanarfogel, led the Ramban to accept the institution of pilagshut (concubines) as an alternative. Married Jewish men have been cheating for rather a long time.